There are some drawings in Ken Hoole's NER locomotives, but to find more detailed stuff, you need to
try and find out where the mods were undertaken, then check out the York or Darlington lists,
or maybe Stratford. Since the class was built again during late LNER/early BR times, it would be
worth looking at lists of GA's for there too.

There are photos in Yeadon's of all four engines -pages 53 and 56. In Hoole's book there are dimensioned drawings of various NER chimneys. From this a possible candidate would be a D17 chimney minus the capuchin, this would give a 7.5inch reduction in overall height not the 10inches quoted in RCTS. However, RTCS also states the reduction brought the chimney to the same height as the dome, which from a visual check of the photo of 68671 may not have been the case. Finally, it should be possible from the photo of 68671 to scale off the approximate size and dimensions.

The J72 chimneys were reduced by 12 inches to get under the bridge at CQ.
Incidentally I thought there were only three of them (2184, 2320 & 2307), but on re-reading my books they were joined by 1715 in 1942.
The chimney height was 11 ft 0.5" above the rail and it is claimed they retained the shape/style of the original.

These were normally employed at Birkenhead Docks but were supposed to replace the J62s on the Buckley line if needed.

Incidentally, Buckly (Old) station might make a good model if space is short.

Try the Buckley Railway (published by the Buckley Society) for lots of pictures of this line.